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<div class="titlepage"><div><div><h4 class="title">
<a name="spirit.advanced.customize.handles_container"></a><a class="link" href="handles_container.html" title="Determine Whether a Component Handles Container Attributes (Qi and Karma)">Determine
        Whether a Component Handles Container Attributes (Qi and Karma)</a>
</h4></div></div></div>
<h6>
<a name="spirit.advanced.customize.handles_container.h0"></a>
          <span class="phrase"><a name="spirit.advanced.customize.handles_container.handles_container"></a></span><a class="link" href="handles_container.html#spirit.advanced.customize.handles_container.handles_container">handles_container</a>
        </h6>
<p>
          The template <code class="computeroutput"><span class="identifier">handles_container</span></code>
          is a template meta-function used as an attribute customization point. It
          is invoked by the <span class="emphasis"><em>Qi</em></span> <a class="link" href="../../qi/reference/operator/sequence.html" title="Sequence Parser (a &gt;&gt; b)">Sequence</a>
          (<code class="computeroutput"><span class="special">&gt;&gt;</span></code>) and <span class="emphasis"><em>Karma</em></span>
          <a class="link" href="../../karma/reference/operator/sequence.html" title="Sequence Generator (a &lt;&lt; b)">Sequence (<code class="computeroutput"><span class="special">&lt;&lt;</span></code>)</a> operators in order to determine
          whether a sequence element (component) handles container attributes directly.
          This customization point is invoked for container attributes only, and
          only if the sequence is compatible with the supplied container attribute.
        </p>
<p>
          If a component, which is part of a sequence is able to handle a container
          attribute directly, the sequence passes the attribute to the component
          without any additional action. In <span class="emphasis"><em>Spirit.Qi</em></span> the component
          uses the attribute to directly store all matched attributes. In <span class="emphasis"><em>Spirit.Karma</em></span>
          the generator component extracts the attributes needed for output generation
          directly from this attribute.
        </p>
<p>
          If a component, which is part of a sequence is not able to handle container
          attributes, in <span class="emphasis"><em>Spirit.Qi</em></span> the sequence passes a new
          instance of the container attributes' <code class="computeroutput"><span class="identifier">value_type</span></code>
          to the parser component, inserting the result into the attribute on behalf
          of the parser component. In <span class="emphasis"><em>Spirit.Karma</em></span> the sequence
          extracts the next container element on behalf of the generator component
          and passing it the extracted value.
        </p>
<h6>
<a name="spirit.advanced.customize.handles_container.h1"></a>
          <span class="phrase"><a name="spirit.advanced.customize.handles_container.header"></a></span><a class="link" href="handles_container.html#spirit.advanced.customize.handles_container.header">Header</a>
        </h6>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">home</span><span class="special">/</span><span class="identifier">support</span><span class="special">/</span><span class="identifier">handles_container</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
          Also, see <a class="link" href="../../structure/include.html" title="Include">Include Structure</a>.
        </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
            This header file does not need to be included directly by any user program
            as it is normally included by other Spirit header files relying on its
            content.
          </p></td></tr>
</table></div>
<h6>
<a name="spirit.advanced.customize.handles_container.h2"></a>
          <span class="phrase"><a name="spirit.advanced.customize.handles_container.namespace"></a></span><a class="link" href="handles_container.html#spirit.advanced.customize.handles_container.namespace">Namespace</a>
        </h6>
<div class="informaltable"><table class="table">
<colgroup><col></colgroup>
<thead><tr><th>
                  <p>
                    Name
                  </p>
                </th></tr></thead>
<tbody><tr><td>
                  <p>
                    <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">traits</span></code>
                  </p>
                </td></tr></tbody>
</table></div>
<h6>
<a name="spirit.advanced.customize.handles_container.h3"></a>
          <span class="phrase"><a name="spirit.advanced.customize.handles_container.synopsis"></a></span><a class="link" href="handles_container.html#spirit.advanced.customize.handles_container.synopsis">Synopsis</a>
        </h6>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span>
    <span class="keyword">typename</span> <span class="identifier">Component</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Attribute</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Enable</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">handles_container</span>
<span class="special">{</span>
    <span class="special">&lt;</span><span class="identifier">unspecified</span><span class="special">&gt;;</span>
<span class="special">};</span>
</pre>
<h6>
<a name="spirit.advanced.customize.handles_container.h4"></a>
          <span class="phrase"><a name="spirit.advanced.customize.handles_container.template_parameters"></a></span><a class="link" href="handles_container.html#spirit.advanced.customize.handles_container.template_parameters">Template
          parameters</a>
        </h6>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                  <p>
                    Parameter
                  </p>
                </th>
<th>
                  <p>
                    Description
                  </p>
                </th>
<th>
                  <p>
                    Default
                  </p>
                </th>
</tr></thead>
<tbody>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">Component</span></code>
                  </p>
                </td>
<td>
                  <p>
                    The component type <code class="computeroutput"><span class="identifier">Component</span></code>
                    which needs to be tested whether it handles container attributes
                    directly.
                  </p>
                </td>
<td>
                  <p>
                    none
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">Attribute</span></code>
                  </p>
                </td>
<td>
                  <p>
                    The attribute type <code class="computeroutput"><span class="identifier">Attribute</span></code>
                    as passed to the sequence operator.
                  </p>
                </td>
<td>
                  <p>
                    none
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">Context</span></code>
                  </p>
                </td>
<td>
                  <p>
                    This is the type of the current component execution context.
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">unused_type</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">Iterator</span></code>
                  </p>
                </td>
<td>
                  <p>
                    The type, <code class="computeroutput"><span class="identifier">Iterator</span></code>
                    is the type of the iterators used to invoke the component.
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">unused_type</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">Enable</span></code>
                  </p>
                </td>
<td>
                  <p>
                    Helper template parameter usable to selectively enable or disable
                    certain specializations of <code class="computeroutput"><span class="identifier">is_container</span></code>
                    utilizing SFINAE (i.e. <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span></code>
                    or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">disable_if</span></code>).
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="keyword">void</span></code>
                  </p>
                </td>
</tr>
</tbody>
</table></div>
<div class="variablelist">
<p class="title"><b>Notation</b></p>
<dl class="variablelist">
<dt><span class="term"><code class="computeroutput"><span class="identifier">Component</span></code></span></dt>
<dd><p>
                A component type to be tested whether it directly handles container
                attributes in the context of sequences.
              </p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">Attribute</span></code></span></dt>
<dd><p>
                A container attribute type as passed to the sequence.
              </p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">T1</span></code>, <code class="computeroutput"><span class="identifier">T2</span></code>, ...</span></dt>
<dd><p>
                Arbitrary types
              </p></dd>
</dl>
</div>
<h6>
<a name="spirit.advanced.customize.handles_container.h5"></a>
          <span class="phrase"><a name="spirit.advanced.customize.handles_container.expression_semantics"></a></span><a class="link" href="handles_container.html#spirit.advanced.customize.handles_container.expression_semantics">Expression
          Semantics</a>
        </h6>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                  <p>
                    Expression
                  </p>
                </th>
<th>
                  <p>
                    Semantics
                  </p>
                </th>
</tr></thead>
<tbody><tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">handles_container</span><span class="special">&lt;</span><span class="identifier">Component</span><span class="special">,</span> <span class="identifier">Attribute</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
                  </p>
                </td>
<td>
                  <p>
                    Result of the metafunction that evaluates to <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span></code>
                    if a given component type <code class="computeroutput"><span class="identifier">Component</span></code>,
                    handles container attributes directly, <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span></code>
                    otherwise. Generally, any implementation of <code class="computeroutput"><span class="identifier">handles_container</span></code>
                    needs to behave as if if was a <a href="../../../../../../../libs/mpl/doc/refmanual/integral-constant.html" target="_top">MPL
                    Boolean Constant</a>.
                  </p>
                </td>
</tr></tbody>
</table></div>
<h6>
<a name="spirit.advanced.customize.handles_container.h6"></a>
          <span class="phrase"><a name="spirit.advanced.customize.handles_container.predefined_specializations"></a></span><a class="link" href="handles_container.html#spirit.advanced.customize.handles_container.predefined_specializations">Predefined
          Specializations</a>
        </h6>
<p>
          <a href="http://boost-spirit.com" target="_top">Spirit</a> predefines specializations
          of this customization point for several types. The following table lists
          those types together with the conditions for which the corresponding specializations
          will evaluate to <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span></code> (see <a href="../../../../../../../libs/mpl/doc/refmanual/integral-constant.html" target="_top">MPL
          Boolean Constant</a>):
        </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                  <p>
                    Template Parameters
                  </p>
                </th>
<th>
                  <p>
                    Semantics
                  </p>
                </th>
</tr></thead>
<tbody>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">Component</span></code>, <code class="computeroutput"><span class="identifier">Attribute</span></code>
                  </p>
                </td>
<td>
                  <p>
                    Always returns <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span></code>
                    (the default).
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">rule</span><span class="special">&lt;</span><span class="identifier">Iterator</span><span class="special">,</span>
                    <span class="identifier">T1</span><span class="special">,</span>
                    <span class="identifier">T2</span><span class="special">,</span>
                    <span class="identifier">T3</span><span class="special">,</span>
                    <span class="identifier">T4</span><span class="special">&gt;</span></code>,
                    <code class="computeroutput"><span class="identifier">Attribute</span></code>
                  </p>
                </td>
<td>
                  <p>
                    Returns <code class="computeroutput"><span class="identifier">is_container</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;</span></code>, where <code class="computeroutput"><span class="identifier">A</span></code>
                    is the attribute exposed by the rule (<span class="emphasis"><em>Spirit.Qi</em></span>
                    and <span class="emphasis"><em>Spirit.Karma</em></span>).
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">grammar</span><span class="special">&lt;</span><span class="identifier">Iterator</span><span class="special">,</span>
                    <span class="identifier">T1</span><span class="special">,</span>
                    <span class="identifier">T2</span><span class="special">,</span>
                    <span class="identifier">T3</span><span class="special">,</span>
                    <span class="identifier">T4</span><span class="special">&gt;</span></code>,
                    <code class="computeroutput"><span class="identifier">Attribute</span></code>
                  </p>
                </td>
<td>
                  <p>
                    Returns <code class="computeroutput"><span class="identifier">is_container</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;</span></code>, where <code class="computeroutput"><span class="identifier">A</span></code>
                    is the attribute exposed by the grammar (<span class="emphasis"><em>Spirit.Qi</em></span>
                    and <span class="emphasis"><em>Spirit.Karma</em></span>).
                  </p>
                </td>
</tr>
</tbody>
</table></div>
<h6>
<a name="spirit.advanced.customize.handles_container.h7"></a>
          <span class="phrase"><a name="spirit.advanced.customize.handles_container.when_to_implement"></a></span><a class="link" href="handles_container.html#spirit.advanced.customize.handles_container.when_to_implement">When
          to implement</a>
        </h6>
<p>
          The customization point <code class="computeroutput"><span class="identifier">handles_container</span></code>
          needs to be implemented for a specific type whenever this type directly
          handles container attributes. It is applicable for parsers (<span class="emphasis"><em>Spirit.Qi</em></span>)
          and generators (<span class="emphasis"><em>Spirit.Karma</em></span>). It will have to be
          implemented under rare circumstances only.
        </p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></td>
</tr></table>
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